BOSTON – Former Plymouth police Sgt. Shawn Coughlin took the witness stand Friday to deny allegations he used excessive force on a defenseless prisoner.

Using a video of his altercation with drunken-driving suspect John Leighton Jr. that was entered into evidence earlier in the week, Coughlin provided his own frame-by-frame explanation for his actions on screen.

He insisted he entered the cell in defense of a fellow officer who was already involved in a heated physical and verbal confrontation with Leighton, and he categorically denied hitting the prisoner with palm or knee strikes.

Coughlin told jurors he did not know Leighton was handcuffed and suffered a kick to his groin early in the scuffle on the night of Nov. 19, 2011. "He was not compliant at all. He's fighting, resisting, twisting, turning. I can't even hold his leg still," Coughlin said of Leighton.

An 18-year police veteran who joined the Plymouth force in 1997, Coughlin said he initially thought Leighton was not handcuffed because he heard the prisoner talking on his cell phone. In fact, Leighton was talking with his hands cuffed behind his back.

Coughlin said he sent Officer Jonathan Yule into the cell to get the phone and then raced to his defense after hearing a heated argument and saw physical contact.

He denied hitting Leighton with his hands, saying he was just trying to hold Leighton's head down. Movements said to be knee strikes he attributed to simple repositioning of his body as Leighton twisted and struggled beneath him.

Coughlin denied kicking Leighton at the end of the incident, explaining he was only rolling the prisoner over to take his weight off his handcuffs.

A day earlier, Charles Key, a police consultant from Baltimore, used the video to pinpoint what he said were numerous instances of Coughlin hitting Leigton with his knee and hand.

Key told jurors that Leighton, who was charged with second-offense drunken driving, two counts of assault and battery on a police officer, two counts of assault and battery with a dangerous weapon and speeding, was compliant throughout most of the attack and that all of Coughlin's behavior constituted excessive force.

Yule told jurors he remembers Coughlin hitting Leighton four to nine times in a matter of seconds, after they hit the floor. Prosecutors suggest the video showed Coughlin continuing to hit Leighton with his knees after Yule left to get a pair of shackles.

Key stopped the video repeatedly to point out individual strikes.

Both Yule and Coughlin were charged internally with trying to cover up the incident by filing false reports.

Yule left the department on his own, and Coughlin was fired. He can still take the case to arbitration.

Prosecutors rested their case early Friday afternoon after calling five final witnesses. Coughlin will retake the stand when the trial resumes Tuesday. Final arguments and deliberations are expected Wednesday.